The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to falling film evaporators for HVAC systems.
HVAC systems, such as chillers, use an evaporator to facilitate a thermal energy exchange between a refrigerant in the evaporator and a medium flowing in a number of evaporator tubes positioned in the evaporator. In a flooded evaporator, the tubes are submerged in a pool of refrigerant. This results in a particularly high volume of refrigerant necessary, depending on a quantity and size of evaporator tubes, for efficient system operation. Another type of evaporator used in chiller systems is a falling film evaporator. In a falling film evaporator, the evaporator tubes are positioned typically below a distribution manifold from which refrigerant is urged, forming a “falling film” on the evaporator tubes, utilizing gravity to drive the flow of refrigerant over the evaporator tubes. Evaporation is primarily accomplished through thin film evaporation on the surface of the evaporator tubes, while a small fraction of refrigerant is boiled off in a pool boiling section of the evaporator.
One of the advantages of gravity feed is that the falling liquid film can be very precisely located such that the risk of maldistribution on the tubes is lowered. The main disadvantage arises from the requirements of gravity feed itself; a stable liquid level needs to be maintained in the distributors such that all of the orifices in the distributor box see the same hydrostatic pressure and deliver the same amount of refrigerant to the tubes below. Furthermore, it is intended that the implementation of falling film technology should not increase the footprint requirements vs. existing flooded products, nor should it increase the amount of liquid refrigerant stored in the distribution system. This limits the height of liquid film that can be used, which in turn increases the possibility that the flow through the orifices will be uneven, since a small change in liquid level height within the distributor (due to, for example, slanted or unlevel installation, or shifting during operation) will have a larger effect than in systems with more generous liquid level allowances.